FIG. 1A shows a schematic cross sectional view of a photoelectric converting apparatus and FIG. 1B is a diagram showing an equivalent circuit of one of the photoelectric converting cells in this apparatus.
In FIG. 1A, an n.sup.- epitaxial layer 4 is formed on an n silicon substrate 1. Photoelectric converting cells which are electrically insulated from one another by a device separating region 6 are arranged in the layer 4.
First, a p base region 9 of a bipolar transistor is formed on the n.sup.- epitaxial layer 4. An n.sup.+ emitter region 15 is formed in the p base region 9. Further, a capacitor electrode 14 to control the potential of the p base region 9 and an emitter electrode 19 connected to the n.sup.+ emitter region 15 are formed through an oxide film 12, respectively.
An electrode 17 connected to the capacitor electrode 14 is formed. An n.sup.+ region 2 for ohmic contact is formed on the back side of the substrate 1. A collector electrode 21 of the bipolar transistor is formed under the n.sup.+ region 2. In this manner, a photoelectric converting cell is constructed.
The fundamental operation of the photoelectric converting cell will now be described. First, the p base region 9 which is biased to the negative potential is set into the floating state. The holes in the electron/hole pairs generated by the light excitation are accumulated into the p base region 9 (accumulation operation).
Subsequently, the positive voltage is applied to the capacitor electrode 14, thereby forwardly biasing the circuit between the emitter and the base. The accumulated voltages generated by the holes accumulated are read out to the emitter side in the floating state (readout operation).
Then, the emitter side is grounded and pulses of the positive voltage are applied to the capacitor electrode 14, thereby extinguishing the holes accumulated in the p base region 9. Thus, when the positive voltage pulse for refreshing falls, the p base region 9 is reset to the initial state (refresh operation).
In such a photoelectric converting apparatus, after the accumulated charges were amplified by the amplifying function of each cell, they are read out, so that the high output, high sensitivity, and low noise can be accomplished. On the other hand, since the structure is simple, this apparatus is also advantageous to realize the high resolution in future.
However, when a photoelectric converting apparatus is constructed by arranging a plurality of photoelectric converting cells mentioned above, there is a problem such that in the case where a saturation light amount or more is irradiated onto a certain pixel, smearing occurs. Namely, when the intense light enters, a large quantity of holes are accumulated in the p base region 9, so that the base potential increases. When the base potential increases to a value in excess of the collector potential, a depletion layer 22 between the base and the collector is extinguished and the accumulated carriers in the base flow into the adjacent cells (as indicated by arrows 23). Thus, the p base region 9 of the adjacent cells enters the accumulation state in which the inflow holes were added. When an image of the readout signal is reproduced, smearing occurs.
On the other hand, when a long sensor is formed using the photoelectric converting cells, in the case of the configuration of the conventional photoelectric converting apparatus, there are problems such that the refresh operation conditions differ in every cell, and this results in the occurrence of the fixed pattern noise, variation in sensitivity, and the like.